SAN FRANCISCO (Reuters) - Arctic ice at the North Pole melted at a record rate in the summer of 2007, the latest sign that climate change has accelerated in recent years, climate scientists said on Wednesday.

"In 2007, we had off-the-charts warming," Michael Steele, an oceanographer at the University of Washington, said at the 2007 meeting of the American Geophysical Union, where 15,000 researchers have gathered to discuss earthquakes, water resources, and climate change.

It was an ominous summer for the Arctic region, where for the first time in recorded history, ships sailed across the Arctic Ocean in water that had been part of the polar ice cap, said Donald Perovich of the U.S. Army Cold Regions Research and Engineering Laboratory in New Hampshire.

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While in the summer of 1980 the North Pole was covered by an ice sheet about the size of the continental United States, this summer the ice would not have covered the states west of the Mississippi River, he added.

"It's a tremendous decrease, but of course, the mystery is how did it happen?" Perovich said.

Scientists said two principal factors are accelerating the vanishing of the polar ice pack, which helps cool the Earth by reflecting the sun's rays back into the atmosphere.

As temperatures in the Pacific and Atlantic Oceans rise, warmer water moves into the Arctic Ocean. This helps melt the polar ice cap, which this year floated in water about 6.3 degrees Fahrenheit (3.5 degrees Celsius) warmer than its historical mean, Steele said.

"Water that is now circulating just 200 meters below the main ice pack is now significantly warmer than it was just five years ago," said John Walsh of the University of Alaska, Fairbanks.

AMPLIFYING SMALL CHANGES

As ice in the arctic melts to water, it also reflects only 7 percent of the sun's radiation, much less than the 85 percent that ice normally reflects. As more of the Arctic Ocean is exposed, it absorbs these extra sun rays, further hastening the planet's rising temperature, Perovich said.

"It's a classic positive feedback. And these feedbacks are important from a climate perspective, because they can take small changes and amplify them," Perovich said.

He said people near the Arctic Circle are already seeing some of the effects of polar warming. Companies are starting to explore for natural resources in newly exposed areas, and coastal villages are grappling with erosion as sea levels rise.

The scientists also expressed skepticism about humans' ability to help generate a cold winter soon enough that could allow the ice cap to refreeze.

New research shows that carbon dioxide, one gas that traps heat in the atmosphere, can be captured as it leaves coal-burning power plants and then permanently sequestered in rock formations thousands of feet below the Earth's surface.

However, it will be about 10 years before the first of such power plants comes online, said Julianna Fessenden of the Los Alamos National Laboratory.

"It's basically the fourth quarter, and we're down two touchdowns," Perovich said. "As you go farther down this (global warming) path, it becomes harder to come back."